Which company built the steam locomotive 'Mallard'?

The London and North Eastern Railway built the steam locomotive Mallard.

Mallard, officially LNER Class A4 number 4468, was built at the LNER’s Doncaster Works and entered service on 3 March 1938. It was designed by Nigel Gresley as a streamlined 4-6-2 Pacific locomotive for fast express passenger trains on the East Coast Main Line.

On 3 July 1938, Mallard reached 126 mph during a test run, establishing the world speed record for a steam locomotive. Its aerodynamic casing, powerful three-cylinder arrangement and high-pressure boiler were central to its performance, although the record run also caused mechanical damage because the locomotive was pushed exceptionally hard.

Mallard was withdrawn in 1963 after covering nearly 1.5 million miles and was preserved rather than scrapped. It is now displayed at the National Railway Museum in York. The locomotive is often confused with Flying Scotsman, another famous British engine; Flying Scotsman is a locomotive, while the name also belongs to a celebrated express passenger service.